1. Field of the Invention
The present invention is generally directed towards methods of sorbing sulfur compounds, particularly H2S, SO2, and organosulfur compounds, from a fluid using mesoporous metal oxide compounds. Metal oxide compounds for use with the present invention include porous compounds having soft Lewis acids impregnated therein or sorbed in the pores thereof, carbon coated metal oxide compounds, and porous nanocrystalline metal oxide compounds which themselves exhibit soft Lewis acid properties. The metal oxide compound is contacted with the fluid containing the sulfur compounds.
2. Description of the Prior Art
Sulfur-containing compounds are present in all fractions of crude oil, some constituting up to 2.5% by weight of the particular fraction. These sulfur-containing compounds can poison many catalysts used in chemical processes. In particular, the Group VIII metal catalysts are extremely sensitive to sulfur poisoning. Also, the generation of sulfur oxides during the combustion of sulfur-containing fuels and the oxidation of these oxides to H2SO4 in automotive exhaust constitutes a major environmental concern to the point that the U.S. Environmental Protection Agency has imposed standards requiring that the maximum sulfur contents of gasoline and diesel fuel be 30 and 15 ppm, respectively, by 2006. These levels are down dramatically from present levels which are as high as several hundred ppm of sulfur compounds.
In oil refineries, an enormous effort is focused on the removal of organosulfur molecules from oil. Generally, such removal is achieved by catalytic processes at high temperatures and pressures. The conventional hydrodesulfurization (HDS) process that is widely used is very efficient for the removal of thiols and sulfides, but is less effective for removal of thiophenes and related derivatives. Therefore, unacceptably high concentrations of organosulfur compounds remain in the fuel stream.
The use of sorbents to remove these remaining portions of organosulfur compounds has been investigated in the past, however no sorbent has been shown to have an enhanced sorption capacity over an extended range of sulfur concentrations and the capability to remove all organosulfur compounds to the desired concentration while being capable of regeneration and production at a low cost.
Generally, the sulfur sorbent materials fall into two categories: (1) chemisorbents which are solid substances that chemically bind sulfur-contaminated compounds, and (2) physisorbents which are solid substances that adsorb the sulfur compounds by weak intermolecular forces, such as van der Waals interaction. Physisorbents, in principle, can work at ambient conditions and have a substantial capacity for removal of sulfur compounds at relatively high concentrations. The main drawback of physisorbents is their inability to reduce sulfur compound concentrations to low levels approaching 15 ppm. Chemisorbents do lower the sulfur content considerably, however the adsorption process must occur at elevated temperatures, about 200°-500° C. and higher. Furthermore, regeneration of chemisorbents is also very difficult and chemisorbents tend not to exhibit the necessary capacity for removing compounds present at high levels.
Combinations of conventional chemisorbents and physisorbents have been suggested to overcome the problems with using purely chemi- or physisorbent materials. However, due to completely different operational temperatures, blended adsorbents demand complicated purification processes which result in higher operational costs. U.S. Pat. No. 5,146,039 discloses the introduction of transition metal ions in a zeolite framework for removal of sulfides and disulfides to levels of 5 ppb at temperatures of 60°-120° C., however, the adsorption capacity for these materials is low. For example, hydrocarbon feeds with sulfur content greater than 20 ppm could not be used with these adsorbents.
As a further illustration of the problems associated with these zeolite compounds, U.S. Pat. No. 5,807,475 describes a zeolite adsorbent (Ni-zeolite-X and Mo-zeolite-X, for example) for thiophene and mercaptan removal from gasoline in the temperature range of 10°-100° C. However, the adsorption capacity is not high, and the sulfur recovery does not exceed 40-50%.
Therefore, there is a real and unfulfilled need in the art for an improved sorbent material which has enhanced sorption capacity over a broad range of sulfur concentrations, has the capability to remove a wide variety of organosulfur compounds, can be easily regenerated, and is cost effective to produce.